Analysis and Design of Passive Polyphase Filters

Passive RC polyphase filters (PPFs) are analyzed in detail in this paper. First, a method to calculate the output signals of an n-stage PPF is presented. As a result, all relevant properties of PPFs, such as amplitude and phase imbalance and loss, are calculated. The rules for optimal pole frequency planning to maximize the image-reject ratio provided by a PPF are given. The loss of PPF is divided into two factors, namely the intrinsic loss caused by the PPF itself and the loss caused by termination impedances. Termination impedances known a priori can be used to derive such component values, which minimize the overall loss. The effect of parasitic capacitance and component value deviation are analyzed and discussed. The method of feeding the input signal to the first PPF stage affects the mechanisms of the whole PPF. As a result, two slightly different PPF topologies can be distinguished, and they are separately analyzed and compared throughout this paper. A design example is given to demonstrate the developed design procedure.